1. Field of the invention
The invention relates to an improved triangulation device which utilizes a tightly focused-projected light spot and a high frequency position sensitive detector. The invention also relates to using such a device to indicate and/or measure a variable of a web such as surface plasticization, surface roughness, surface gloss, coating quality, surface density, mean distance between fibers of a paper web, ash content of a paper web surface, etc. The invention also relates to a method of measuring a thickness of a web, and specifically a moving web, using a triangulation device arranged on both sides of the web.
2. Description of the Prior Art
Optical triangulation is accomplished by a device which projects a small spot of light onto a surface. A lens, whose optical axis is at an angle to the axis of the projected light, forms an image of the spot onto a position sensitive detector. If the surface moves closer or farther from the device, the image of the spot moves as a result. The output of the position sensitive detector is used to determine the distance from the device to the surface.
The thickness (or caliper) of a sheet can be measured with two optical triangulation devices located on opposite sides of the sheet and arranged at a known distance apart. Each device measures the distance to one side of the sheet and the sum is subtracted from the known distance apart in order to obtain the caliper.
Single sided applications for thickness measurement are possible, i.e. if the web is supported by a roll, the optical device measures the distance to the paper surface. This distance is subtracted from the known the distance to the roll to determine thickness.
Caliper is an important measurement in papermaking. The industry standard caliper sensor is an electromechanical device. A ferrite disc touches one side of the sheet while a coil with a ferrite U-core touches the other side. The coil is part of the resonant circuit of an oscillator. When the caliper of the sheet changes the distance between the ferrite disc and the U-core changes. This shifts the resonant frequency of the oscillator. This type of sensor is capable of sub micron accuracy and is widely used all over the world.
There are disadvantageous with using a device with parts which touch the sheet, i.e., with a device which uses contact measurement. The main disadvantage of the device is that on fast moving sheets, such contact can produce holes or tears. These holes or tears occur when an imperfection in the sheet, such as grit, hits the caliper sensor. Usually, this tearing has the form of a triangular shaped hole with the apex at the imperfection. Then, if paper sheet with a hole reaches, e.g., a printing press, the hole can cause a costly sheet break that typically stops the press for several hours. As more recycled products are used in papermaking, imperfections become a growing problem. An accurate non-contacting caliper sensor would therefore solve the growing problem of holes or tears caused by contact measuring devices.
Besides these holes or tears, the contacting elements of the measuring device also can cause marks on a sheet. These marks are typically shiny streaks formed on the surface of the sheet due to a “polishing” effect caused by the contacting sensor elements. These more polished surface areas can be responsible for uneven color application during,.e.g., a printing process. As a result, contacting caliper measurement is not used on sensitive paper grades.
Optical triangulation has been used for non-contacting measurement of paper caliper. One factor, which limits the accuracy of this measurement, however, is the scattering of light into the paper. Paper is generally a mat of wood fibers bonded together. As light scatters between the fibers, the projected light spot diffuses into the paper forming a larger spot (i.e., larger than the spot initially projected onto the surface) centered below the surface of the paper. Since paper fibers are randomly distributed, the relationship between the center of the projected light spot and the surface of the paper is inconsistent. This inconsistency contributes to error when compared to a mechanical caliper measurement technique.
Scattering will cause the non-specularly reflected spot size from paper to be approximately 50 to 100 μm in diameter regardless of how small the incident beam is made. As a result, it is very difficult to determine the spot position with an accuracy that is adequate for the required 0.25 μm measurement resolution.